Automobile NVH (i.e., noise, vibration, and harshness) has gained interest recently with rising consumer expectations. In the past, engines, transmissions and tires were of the most concern to researchers, but as these became quieter, and with the advent of electric vehicles, other sources of NVH on a vehicle have become more significant.
In an effort to meet the increasing demands of vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved vehicle stability management (VSM) features. One such feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems include vehicle wheel speed sensors for providing inputs to an anti-lock brake system control unit. The control unit controls anti-lock brake system control valves interposed between the brake master cylinder and the individual wheel brakes of a hydraulic brake circuit. Such control valves include isolation valves and dump valves. The control valves, in turn, regulate or modulatel hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking and/or dynamic rear proportioning. One or more ABS pumps pump the fluid from one or more low pressure accumulators (LPA) to the control valves or back to the master cylinder.
In operation, one or more of the vehicle wheel speed sensors not only senses the vehicle wheel speed, but also provides input to the control unit for determining the vehicle speed. The control unit monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity and the vehicle velocity.
Next, the control unit compares the vehicle wheel velocity to the vehicle velocity to determine a departure depth. Again, using the vehicle velocity as a reference, departure depth may be expressed as the difference between the vehicle velocity and the wheel velocity. During normal vehicle braking, the wheel velocity closely matches the vehicle velocity. Thus, during normal vehicle braking, the difference between the vehicle velocity and the wheel velocity is nominal.
However, during an anti-lock braking event, the wheel velocity decreases significantly below, or "departs" from, the vehicle reference velocity. This is called "departure". In such a situation, as for example during hard braking on an ice covered road, the frictional force between the vehicle brake pads and the vehicle wheel exceeds that between the vehicle wheel and the road surface. Uncontrolled, such a frictional force differential causes the vehicle wheel to cease rotating, or to "lock".
In turn, locking causes the vehicle wheels to slip or "skid", rather than roll, over the road surface. Such vehicle wheel skidding dramatically reduces traction and the ability of the vehicle operator to bring the vehicle to a controlled stop.
To prevent such vehicle wheel lock and the accompanying problems, the control unit of an anti-lock brake system activates the anti-lock brake system isolation valves to regulate hydraulic brake fluid pressure in the individual wheel brakes during an anti-lock braking event. Typically, there is both a wheel slip and wheel deceleration criterion required for the activation of the isolation valves. More specifically, the control unit compares the departure depth to the slip threshold and actuates the isolation valves when the departure depth exceeds the slip threshold in order to isolate the individual vehicle wheel brakes in the hydraulic brake circuit from the master cylinder, thereby halting any increase in brake fluid pressure in the vehicle wheel brakes and preventing incipient vehicle wheel lock.
More particularly, when, during vehicle braking, the departure depth exceeds the slip threshold, each isolation valve isolates brake fluid in its individual wheel brake from the increasing brake fluid pressure in the master cylinder in order to hold brake fluid pressure in the wheel brake constant. If the isolated brake fluid pressure in the wheel brake is still high enough to cause incipient wheel lock, the anti-lock brake system then bleeds, or dumps, brake fluid from the wheel brake through its dump valve to reduce brake fluid pressure therein. The dumped brake fluid then flows back to the accumulator.
Thereafter, the anti-lock brake system typically holds brake fluid pressure in the wheel brake constant until such time as the departure depth no longer exceeds the slip threshold, indicating that the vehicle wheel is again traveling at or near the velocity of the vehicle. At that time, the anti-lock brake system then increases, or builds, brake fluid thereto. Reapplication of brake fluid to the wheel brake may be at a steep or gradual rate, or some combination thereof, depending upon the circumstances or the control desired.
To maintain smooth braking and optimum vehicle control, some reapplication of brake fluid to the wheel brakes must be undertaken where the isolation of the brake fluid in the wheel brakes from that in the master cylinder has been prolonged, for example on the order of one hundred milliseconds or greater. Such reapplication must be undertaken in order to raise brake fluid pressure in the wheel brake to a level approximately that in the master cylinder before the isolation valve may desolate the wheel brake back to master cylinder pressure.
Most ABS systems use a motor-driven pump to move or pump brake fluid from a low-pressure storage source or area (i.e. accumulator) to a high-pressure source (i.e. main brake line). In this way, pressure can be increased in the brakes, allowing modulation of brake pressure to control brake torque.
Current technology for ABS systems typically turns the pump motor fully "on" anytime there is a recognized need to move fluid from the low pressure storage area. The pump motor and the action of pumping fluid within the hydraulic system is a recognized source of audible noise in an ABS system especially when the load on the pump is relatively low and when the pump is to be "on" only a short period of time (i.e., the pump is to be turned "off" at essentially the time the pump has achieved its maximum operating speed).
Minimization of system NVH is a goal of many ABS systems. For example, one approach taken by the prior art to minimize system noise is to provide a closed-loop motor control system for the pump as illustrated in U.S. Pat. No. 4,892,364. However, this requires the added cost of a sensor.
Another approach known by the prior art is to measure the voltage generated by the pump motor to control the pump motor as illustrated in WO 94/07717. However, this approach also requires a feedback signal and control logic to process the feedback signal.